brbread:

Watch out, it’s comin’ Atchafalaya! An unlikely title for a serious issue:

Every thousand years or so, the lower Mississippi changes course.  It piles up enough silt at its delta that it ’spills over’ to a new shortest path to the ocean. At times, the outlet has been anywhere from Texas to the Florida Panhandle.

Since the early 20th century, the Mississippi has been trying to change course again—sending its main flow down the Atchafalaya river, which offers a much shorter, steeper path to the ocean.  The Army Corps of Engineers was ordered by Congress to keep that from happening.  The center of their effort is the Old River Control Structure, which limits the flow down the Atchafalaya to 30%.

Every now and then there’s a massive flood which stresses the system. The fear is that if the Mississippi ever broke through the ORCS and the main flow was captured by the Atchafalaya, it would be very hard or virtually impossible to return it to its old route. This would devastate the people and industries around in Baton Rouge and New Orleans who depend on the river (as if they haven’t had enough problems lately).  This almost happened in 1973, when a massive flood undermined the structure; this was the subject of John McPhee’s book.